Age and sex affect protein metabolism at protein intakes that span the range of adequacy: comparison of leucine kinetics and nitrogen balance data

Research suggests that changes in leucine oxidation (leu_ox) with feeding may reflect adult protein requirements. We evaluated this possibility by assessing the effects of age, sex, and different protein intakes on whole-body leucine kinetics and nitrogen balance. Thirty-four young (n= 18, 22–46 years) and old (n= 16, 63–81 years) men and women completed three 18-day trials with protein intakes of 0.50, 0.75 and 1.00 g protein·kg body weight^? 1·d^? 1. Fasting and fed-state leucine kinetics were quantified on day 12 of each trial using a primed, constant infusion of L-1-13C]leucine. Protein requirement was estimated using classical nitrogen balance measurements and calculations. Leucine kinetics parameters were influenced by age and sex across all protein intakes. With feeding, leu_ox increased more in old vs. young adults. Independent of age, fasting and fed-state leu_ox were lower, and net leucine balance (fasting+fed-state) was higher in women vs. men. Among all subjects and protein intakes, nitrogen balance was correlated with fed-state leu_ox (r= 0.39), fed-state leucine balance (r= 0.60), net leucine balance (r= 0.49) and the change in leu_ox from the fasting to fed state (r= 0.49) (P<.05 for all results). At the highest protein intake, the change in leu_ox with feeding was inversely correlated with protein requirement (r=?0.39). These findings indicate that leucine kinetics, especially leu_ox, reflect nitrogen balance-based estimates of the need for dietary protein and generally support the view that protein requirement is comparable between young and old adults.